Clemens Ritter

High-symmetry lattices of transition metal ions such as the body-centred tetragonal (bct) arrangement are model systems for correlated electron physics and frustrated magnetism. Transition metal oxides or halides with the bct K ₂ NiF ₄ -type structure are extensively studied and have ground states dominated by strong interactions within the square layers, but materials exemplifying other bct ground states are much rarer. Here we show that Ca ₂ CoN ₂ , from the underexplored class of transition metal nitrides, has one of the complex magnetic ground states predicted for the bct lattice lying close to a spin-liquid instability. Ca ₂ CoN ₂ has been synthesised at high pressure in a large-volume press, and has a Na ₂ HgO ₂ -type structure where weakening of intralayer superexchange interactions leads to frustration. Our magnetisation and neutron diffraction experiments reveal an incommensurate spin order below 54 K, consistent with a double- k spin density wave with propagation vectors ( k 00) and (0 k 0) where k = 0.57. This phase lies close to the predicted crossover (at k = 0.64–0.69) to a modulated spin liquid, which could be explored through further chemical tuning. This work demonstrates the potential of transition metal nitrides as magnetic model systems, alongside the much-studied oxides and halides, for exploring complex spin orders and associated quantum phases.

γ − Ba 3 CoNb 2 O 9 realizes a disordered simple-cubic spin- 1 / 2 lattice in which Co 2 + ions randomly occupy one-third of the sites, placing the system just above the site-percolation threshold for magnetic order. Specific-heat, susceptibility, neutron spin-echo, and muon spin-rotation measurements reveal a broad thermodynamic crossover, short-range magnetic correlations, and persistent fast spin dynamics down to at least 0.1 K, with no evidence for static order or conventional spin-glass freezing within the experimental time windows. Monte Carlo simulations yield a broad distribution of orphan spins, finite clusters, and an infinite network. The weakly correlated spin fraction ( ≈ 8.2 % ) inferred phenomenologically from magnetization is consistent with the calculated orphan-spin fraction ( ≈ 8.8 % ). Exact diagonalization of a diluted S = 1 / 2 Heisenberg model captures the broad magnetic specific-heat anomaly and supports the coexistence of weakly and strongly correlated spin environments. These results support a picture in which spin- 1 / 2 quantum fluctuations, together with dilution and proximity to the percolation threshold, can support a disorder-driven dynamical state with short-range correlations in three dimensions, distinct from both classical spin glasses and geometrically frustrated quantum spin liquids.

Anion order in heteroanionic compounds may induce electrical dipoles and the formation of noncentrosymmetric polar structures. The majority of perovskite oxynitrides are centrosymmetric, but some of them show high dielectric permittivities and ferroelectric response resulting from local anion order. Here, we report a study of polar, partially anion-ordered Nd2AlO3N and Sm2AlO3N n = 1 Ruddlesden–Popper oxynitrides, prepared by solid-state reaction in flowing N2 at 1500 °C. They crystallize in the noncentrosymmetric I4mm space group with parameters a = 3.703314(3) and c = 12.545382(14) Å for Nd2AlO3N and a = 3.674477(5) and c = 12.40414(3) Å for Sm2AlO3N. Neutron powder diffraction data of Nd2AlO3N indicate that oxygen occupies the equatorial sites 4b of the AlO5N octahedra, whereas both anions occupy two crystallographically independent 2a axial X2 and X3 sites, with different O/N populations (68/32 for X2 and 32/68 for X3) and a larger bond distance to the Al atom for the oxygen rich site. Dielectric permittivities of 260 and 150 are measured for Nd2AlO3N and Sm2AlO3N, respectively, and second-harmonic generation spectroscopy and scanning transmission electron microscopy furnish complementary evidence of the polar nature of both compounds. These results represent the first experimental demonstration of acentric functionalities in a Ruddlesden–Popper oxynitride.

Ce 2 Te 5 is a layered f -electron van der Waals magnet in which reduced dimensionality and inequivalent Ce sites give rise to competing magnetic interactions. We investigate its magnetic ground state using muon spin relaxation ( μ SR ), neutron powder diffraction (NPD), and inelastic neutron scattering (INS). Zero-field μ SR reveals an onset of static magnetism below T C = 5.0 ( 1 ) K , followed by an additional anomaly in the internal field at T 2 = 2.3 ( 2 ) K , consistent with features observed in bulk thermodynamic and transport measurements. In contrast, NPD data collected between 0.05 – 8 K reveal a single long-range ordered magnetic phase below T C , with the magnetic Bragg intensities vanishing at T C with no evidence for additional structural or magnetic phase transitions down to base temperature. The ordered state is characterized by a commensurate propagation vector k = ( 0 , 0 , 0 ) and ferromagnetic alignment of Ce moments along the crystallographic b axis. Remarkably, only one of the two crystallographically distinct Ce sites carries an ordered moment of ≈ 0.85 ( 3 ) μ B per Ce at 0.05 K. INS measurements establish the crystal-electric-field (CEF) energy scale of Ce 3 + , revealing low-lying excitations at 7.94 and 21.46 meV and a Kramers-doublet ground state with strong single-ion anisotropy. These results demonstrate that Ce 2 Te 5 undergoes a single symmetry-breaking magnetic transition, while additional low-temperature anomalies reflect subtle modifications of the ordered state driven by competing interactions and CEF effects.

Cryptomelane is a hollandite-like material consisting of K+ cations in an α-MnO2 tunnel-like crystallographic motif. Cryptomelane with stoichiometry KxMn8O16 (x ≈ 1.45) has been synthesized and its magnetic properties investigated using variable-temperature magnetic susceptibility, heat capacity, and neutron powder diffraction. Three distinct transitions at T1 = 184 K, T2 = 54.5 K, and T3 = 24 K are observed. At T1 there is a subtle tetragonal → monoclinic transition associated with emergence of a set of nonmagnetic superstructure peaks indexable to a k→struc≈0.74c*→ incommensurate modulation parallel to the α-MnO2 tunnels. Our findings are consistent with a relation previously reported in titanate hollandites, that x≈2|k→struc|. Magnetic Bragg peaks emerge below T2 = 54.5 K, and their positions indicate an incommensurate modulated magnetic structure. The model consistent with the data is a dual- k→mag structure with a ferromagnetic |k→mag|=0 component and an incommensurate k→mag≈0.37c*→, with the latter most likely to be helical. The period of oscillation of the incommensurate magnetic component is in line with predictions based on a Heisenberg spin Hamiltonian [Mandal et al. Phys. Rev. B 2014, 90, 104420]. Below T3 = 24 K, there is a magnetic transition, which gives rise to a different set of magnetic Bragg peaks indicative of a highly complex magnetic structure.

Chemical substitutions offer a universal approach to tailor material properties, but the resulting chemical disorder can impose important structural responses that remain poorly understood. Here we perform a detailed investigation of the effect of chemical disorder in a low-symmetry prototype system A = Ca and Ba in Sr7-xAxMn4O15, through a combined high resolution X-ray and neutron powder diffraction study and first-principles density functional theory (DFT) simulations. We find that, despite the coordination environment across the 4 symmetry-unique Sr sites in the monoclinic P2_1/c structure being nominally the same, a large anisotropic and asymmetric solubility preference is observed for Ca2+ and Ba2+ which have significantly smaller and larger ionic radii, respectively. We contrast this anisotropic control parameter with both experimental and computational studies of the effect of isotropic high-pressure on the evolution of the structural parameters. We show that the complex evolution of lattice parameters (strain) in this series of compounds can only be understood to arise as a result of the coupling between anisotropic site substitution with the acoustic instabilities in this material. Mechanistically, this coupling is shown to be mediated by rigid unit modes, whose composition-dependent evolution accurately reproduces the non-trivial evolution of the lattice parameters across the entire series. Since the existence of coordination polyhedra is rather ubiquitous across functional solid state materials chemistry, the clarification of the interplay between chemical disorder, acoustic instabilities and rigid unit mode distortions represents an important step in the development of approaches for the targeted design of functional materials properties.

The room-temperature structure of triuranium octoxide (U3O8) was investigated by combining neutron total scattering with high-resolution neutron powder diffraction to compare its average and local structural organization. Rietveld refinement of the diffraction data confirms that the long-range structure is well-described by the established orthorhombic model (C2mm) without clear structural expression of uranium charge disproportionation. In contrast, analysis of the pair distribution function derived from the total scattering data indicates that the short-range structure is better described by a monoclinic distortion which remains closely related to the primitive cell of the orthorhombic C2mm phase. Within this lower-symmetry framework, the uranium sites separate into distinct coordination environments: two remain similar to one another, whereas the third exhibits a modified geometry characterized by shortened planar U–O bonds. Bond valence sum analysis of the coordination shells yields one U(VI)-like site and two U(V)-like sites, in agreement with prior spectroscopic observations, ab initio calculations, and theoretical treatments of mixed valence in U3O8. These results support the view that the structural articulation of charge disproportionation in U3O8 is expressed more clearly at the local than at the average structural level and illustrates the value of combining diffraction and total scattering approaches for mixed-valence oxides.

Layered Ruddlesden–Popper (RP) oxides provide a versatile platform for topochemical anion modification, enabling access to metastable mixed-anion phases with tuneable structural and electronic properties. In this work, we present a comprehensive investigation of how mixed B -site (indium and antimony) chemistry influences topochemical fluorination of Ba ₂ In _0.5 Sb _0.5 O ₄ using polyvinylidene fluoride (PVDF). Powder X-ray diffraction (PXRD) reveals anisotropic lattice expansion, dominated by elongation perpendicular to the perovskite layers, with minimal change in the in-plane lattice parameters. Fluorination proceeds through a composition-dependent multiphase regime in which several tetragonal ( I 4 /mmm ) RP components with progressively expanded c and slightly contracted a lattice parameters coexist, representing a distribution of fluorination states, followed by convergence to a single dominant fluorine-rich tetragonal phase at high fluorine content. Coupled X-ray/neutron Rietveld refinement establishes the end member as the anion-saturated oxyfluoride Ba ₂ In _0.5 Sb _0.5 O ₂ F ₄ , and locates the three fully occupied anion sites of the tetragonal I 4 /mmm framework. The preferred O/F distribution, with oxide at the equatorial site and fluoride at the apical and interstitial sites, is supported by bond-valence-sums (BVSs) analysis of the refined interatomic distances and the corresponding global instability indices (GIIs), together with charge balance, since O and F cannot be reliably distinguished from their neutron scattering lengths alone. X-ray absorption near-edge structure (XANES) measurements support the retention of the formal In ³⁺ and Sb ⁵⁺ oxidation states upon fluorination.

La _0.5 Sr _3.5 Fe ₃ O _7.5 F _2.6 is one of the few n  =   3 members of Ruddlesden–Popper oxyfluorides. This compound was recently found to be an antiferromagnet with T _N being well above room temperature. In this contribution, temperature‐dependent neutron diffraction and Mößbauer spectroscopy were applied to get detailed information on the nuclear and magnetic structures of La _0.5 Sr _3.5 Fe ₃ O _7.5 F _2.6 . The nuclear structure was refined in the orthorhombic space group Cmce (No. 64), while for the magnetic structure space group P 2 ₁ /c (No. 14.75) was derived. The antiferromagnetic ordering of the iron moments was found to persist in the very wide temperature range of 1.7–470 K without any structural transitions. While neutron diffraction revealed the loss of long‐range antiferromagnetic ordering to occur above 470 K, Mößbauer spectroscopy showed that short‐range ordering remains up to 525 K.

Layered Ruddlesden–Popper oxides provide a versatile platform for topochemical anion modification, enabling access to metastable mixed-anion phases with tuneable structural and electronic properties. In this work, we present a comprehensive investigation of how mixed B-site (indium and antimony) chemistry influences topochemical fluorination of Ba ₂ In _0.5 Sb _0.5 O ₄ using PVDF. Powder X-ray diffraction reveals anisotropic lattice expansion, dominated by elongation perpendicular to the perovskite layers, with minimal change in the in-plane lattice parameters. Fluorination proceeds through a composition-dependent multiphase regime in which several tetragonal ( I 4 /mmm ) RP components with progressively expanded c and slightly contracted a lattice parameters coexist, representing a distribution of fluorination states, followed by convergence to a single dominant fluorine-rich tetragonal phase at high fluorine content. Coupled X-ray/neutron Rietveld refinement establishes the end member as the anion-saturated oxyfluoride Ba ₂ In _0.5 Sb _0.5 O ₂ F ₄ , and locates the three fully occupied anion sites of the tetragonal I 4 /mmm framework. The preferred O/F distribution, with oxide at the equatorial site and fluoride at the apical and interstitial sites, is supported by bond-valence-sums (BVSs) analysis of the refined interatomic distances and the corresponding global instability indices (GIIs), together with charge balance, since O and F cannot be reliably distinguished from their neutron scattering lengths alone. XPS confirms that In remains in the +3 oxidation state, while the Sb spectral changes are consistent with modifications of the local O/F coordination environment rather than changes in the formal oxidation state. X-ray absorption near-edge structure measurements further support the retention of the formal In ³⁺ and Sb ⁵⁺ oxidation states upon fluorination.